Heavy current D.C. series motors are generally fitted with commutating interpoles to prevent or reduce sparking at the brushes with load changes such as would occur in driving an electric vehicle.
Interpoles are effective for this purpose but they not only increase the cost but require a larger stator to provide room for them between the main poles thus making the motor larger and heavier than would otherwise be necessary. Reducing the cost and weight is important for all motors but is doubly important for the counter rotating motor described in my U.S. Pat. No. 4,130,172 wherein the stator or field structure rotates at high speed and it is imperative to keep its kinetic energy or flywheel effect down to the barest minimum for satisfactory operation.